Self assembly of the transmembrane domain promotes signal transduction through the erythropoietin receptor

Hematopoietic cytokine receptors, such as the erythropoietin receptor (EpoR ), are single membrane-spanning proteins. Signal transduction through EpoR is crucial for the formation of mature erythrocytes [1]. Structural evidenc e shows that in the unliganded form EpoR exists as a preformed homodimer in an open scissor-like conformation [2] precluding the activation of signali ng. In contrast to the extracellular domain of the growth hormone receptor (GHR) [3], the structure of the agonist-bound EpoR extracellular region sho ws only minimal contacts between the membrane-proximal regions CB. This evi dence suggests that the domains facilitating receptor dimerization may diff er between cytokine receptors, We show that the EpoR transmembrane domain ( TM) has a strong potential to self interact in a bacterial reporter system. Abolishing self assembly of the EpoR TM by a double point mutation (Leu 24 0-Leu 241 mutated to Gly-Pro) impairs signal transduction by EpoR in hemato poietic cells and the formation of erythroid colonies upon reconstitution i n erythroid progenitor cells from EpoR(-/-) mice, Interestingly, inhibiting TM self assembly in the constitutively active mutant EpoR R129C abrogates formation of disulfide-linked receptor homodimers and consequently results in the loss of ligand-independent signal transduction. Thus, efficient sign al transduction through EpoR and possibly other preformed receptor oligomer s may be determined by the dynamics of TM self assembly.